Though this is again a little bit late, I couldn't very well let Cinco de Mayo pass without making a blog post about it!
A few things about Cinco de Mayo, or the Fifth of May, are commonly misunderstood. For instance, Cinco de Mayo is not a celebration of Mexican independence. Furthermore, Cinco de Mayo is actually a relatively minor holiday in Mexico, and is celebrated mainly within the United States as a symbol of Mexican culture and heritage.
The holiday instead celebrates the victory of Mexican troops over French during the Battle of Puebla, which was part of what is sometimes called the Franco-Mexican War. When in 1861, Mexico claimed temporary cessation of foreign debt repayment, the British, French, and Spanish all invaded the country. However, in 1862, both the English and Spanish withdrew. The French forces, under Napoleon III, remained in hopes to bring Mexico under the rule of Austrian monarch Maximilian and to defy growing US power in North America.
The Battle of Puebla was a battle on May 5, 1862, in which the Mexican troops enjoyed an unlikely victory over the French. Over 1000 Frenchmen died, and though the war and French occupation continued until 1867, the victory at Puebla came to represent Mexican pride in uprising against foreign oppression.
As for celebration of the holiday today, most celebrations occur in the southwest, where a greater proportion of the population are of Mexican descent. Mexican cultural and religious symbols such as the Virgin de Guadalupe enjoy special recognition on this day. Commercially, many business provide more Mexican goods to their customers. Some schools prepare lessons and banners to inform students about Mexican history. And in some cities, such as Pueblo de los Angeles, the crowd performs and celebrates regional Mexican music and dancing.
Sources: History.com, Britannica.com, Wikipedia, Time And Date.com